1. Field of the Invention
The present invention relates to a knocking detecting sensor attached to an internal combustion engine to detect knocking electrically.
2. Description of the Related Art
In order to detect knocking generated in an internal combustion engine of a motor vehicle, a sensor to detect vibration due to the knocking using a piezoelectric element has been widely used. Such knocking detecting sensors are classified into two kinds: a resonant type and a non-resonant type. The former type has a resonance point in a band corresponding to a knocking frequency and selectively detects only the vibration in the vicinity of the resonating frequency on the basis of a resonance phenomenon, thereby detecting the knocking. The latter type has a comparatively flat output characteristic without a specified resonance frequency and extracts the sensor output in a frequency band corresponding to knocking by using a bandpass filter, thereby detecting the knocking.
Generally, in the non-resonant type of knocking detecting sensor, a main metallic sleeve is arranged so that its bottom face contacts an attaching face on an attaching unit, and a screw member is inserted into a through hole of the main metallic sleeve and is screwed into a female screw hole in the side of the attaching unit to fix the main metallic sleeve and the attaching unit together. (See for example, TOKU HYO HEI 6-508920 and U.S. Pat. No. 5,398,540) Further, there is another type of sensor in which a male screw portion is integrally formed with a main metallic sleeve. This integrated body is screwed to a female screw hole in an attaching side. (See for example, Unexamined Japanese Patent Publication No. HEI. 1-159467). In that attachment, however, it is well known that the main metallic sleeve will vibrate (as seen from the attaching face, the vibration includes an amplitude component in a direction intersecting the attaching face) in a direction along an axial line of the screw member (namely, the coupling direction of the screw member). The vibration will occur around the outer edge portion of the main metallic sleeve when the axial line of the screw member is the center, and will resonate at a specified frequency. It is desirable for the non-resonant type knocking detecting sensor to have a flat output frequency characteristic. However, when resonance occurs as described above, the detected output level in a knocking band should be lowered, or otherwise the output frequency characteristic is susceptible to the influence of noise from the resonance, which leads to erroneous detection. Also, in the resonant type of knocking detecting sensor, resonance may occur in an undesired frequency by a similar or other cause, in which case, erroneous detection is likely to occur.